Method for making and using chromium iii salts

ABSTRACT

A method of preparing an aqueous composition of a chromium III compound, comprising adding hydrogen peroxide to a mixture comprising water and a chromium VI compound in the presence of at least one acid according to the formula H 2 GF 6 , in which G is a Group IV-B element. The composition may contain less than 500 ppm of alkali metal ions and less than 200 ppm of halide ions, relative to chromium and may test negative for chromium VI using s-diphenylcarbazide. The composition may be used for treating a metal surface, among other applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/474,960, filed May 29, 2009, the disclosure of which isincorporated herein by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

Hexavalent chromium compounds have been used as traditional conversioncoatings to treat metal surfaces to improve their corrosion resistanceand paint adhesion. Hexavalent chromium shows toxicological effects andhas been determined by the Environmental Protection Agency as a risk tothe environment and by the Occupational Safety and Health Agency as ahealth risk. Moreover, chemistries based on hexavalent chromium areclassified as carcinogens by these agencies.

It is highly desirable to provide coatings and processes free ofhexavalent chromium but still capable of providing paint adhesion andcorrosion resistance comparable to that provided by conventionalhexavalent chromium-based coatings to metal surfaces. Within the pastfew decades, various compositions and processes not relying onhexavalent chromium have been described and used for treating metalsurfaces. Some of these are based on at least partial replacement ofhexavalent chromium with trivalent chromium.

Known uses of partially reduced chromium, i.e. compositions containingsome trivalent chromium and some hexavalent chromium, include acidulatedrinses for phosphated metal surfaces. These acidulated rinse solutionsmay contain a partially reduced hexavalent chromium solution, i.e. asolution wherein some of the chromium is trivalent. Partially reducedchromium solutions may be manufactured by adding an organic compoundsuch as formaldehyde, methanol, ethanol, etc. to a solution of chromicacid. The chromic acid oxidizes the organic compound, and is itselfreduced in the process. The organic compound(s) must generally be addedvery carefully and slowly to the chromic acid solution, because theresultant heat, boiling and generation of CO₂ can be so energetic as tobe dangerous. Yet these materials are seldom taken to more than 30%reduction of chromium VI to chromium III unless excess mineral acid isadded to help hold the trivalent chromium in solution. Efforts to morecompletely reduce the chromium VI typically result in sludge formation,and full conversion to trivalent chromium is not achieved by suchmethods. An exemplary partially reduced (typically about 30% reduced)chromic acid is available from Bulk Chemicals, Inc. of Reading, Pa.under the trade name BULK RINSE® 6A.

The incomplete conversion of chromium VI to chromium III in these andother processes is undesirable in some applications, and thus, methodsof producing fully reduced chromium salts would be of benefit to theindustry.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method of preparing an aqueouscomposition of a chromium III compound. The method includes addinghydrogen peroxide to a mixture including water and a chromium VIcompound in the presence of at least one acid according to the formulaH₂GF₆, in which G is a Group IV-B element.

In another aspect, the invention provides an aqueous composition thatincludes dissolved Cr₂(GF₆)₃, in which G is a Group IV-B element, andcontains less than 500 ppm of alkali metal ions and less than 200 ppm ofhalide ions, relative to chromium and which tests negative for chromiumVI using s-diphenylcarbazide.

In a further aspect, the invention provides a method of treating a metalsurface. The method includes contacting the metal surface with anaqueous composition including dissolved Cr₂(GF₆)₃, in which G is a GroupIV-B element, and containing less than 500 ppm in total of alkali metalions and less than 200 ppm of halide ions, relative to chromium.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to chromium III salts essentially free ofcontaminating ions, i.e., essentially free of ions that are not part ofthe canonical formula of the salt, and methods for producing and usingsuch salts. These salts will be referred to herein as “high-purity”chromium salts, with the understanding that trace amounts ofcontaminating ions may be present, typically as a result of impuritiesin the ingredients. The methods of this invention are suitable formaking a variety of high-purity chromium III salts, including but notlimited to salts of Group IV-B fluoacids such as chromium fluozirconate,chromium fluotitanate, and chromium fluohafnate. In the case of GroupIV-B salts, it is currently believed that the chromium III salts are atleast approximately of the formula Cr₂(GF₆)₃, with G representing theGroup IV-B metal. For simplicity, the inventors will describe theinvention with respect to chromium III fluozirconate, but it will beunderstood that analogous procedures may be used to make otherhigh-purity chromium III salts as well.

Chromium fluozirconate prepared according to the invention providesexcellent corrosion protection on metal surfaces, including at least Al,Zn and alloys of either of these, and may also be useful for providing apassivation layer for electroplating.

Preparation of High-Purity Chromium III Fluozirconate

The method of making high-purity chromium fluozirconate includesreducing aqueous chromic acid and/or partially reduced aqueous chromicacid with hydrogen peroxide, which is itself oxidized in the process toform gaseous O₂. If chromic acid is used, the reaction is believed toproceed according to the equation shown below, although the inventors donot intend to be bound by this proposed explanation. The skilled artisanwill recognize that CrO₃ is the anhydrous form of chromic acid and isused in the following equation for simplicity.

2CrO₃+3H₂O₂+3H₂ZrF₆→Cr₂(ZrF₆)₃+6H₂O+3O₂↑

During the reaction and afterwards, the mixture remains essentiallyfully liquid and essentially free (i.e., only trace amounts, if any) ofprecipitates or suspended solids. The fluozirconic acid may be addedsimultaneously with the hydrogen peroxide, or may be admixed with thechromic acid prior to adding the hydrogen peroxide, or a combination ofthese.

If partially reduced chromic acid is used as the starting material, insome embodiments, it is prepared by reaction of chromic acid with anorganic compound (for example, formaldehyde or methanol) which isoxidized by the chromic acid and results in partial reduction of thechromium.

In practice, more than a stoichiometric amount of hydrogen peroxide mayin some cases be needed to achieve full reduction of chromium VI tochromium III. This may be the result of decomposition and/orvolatilization of some amount of hydrogen peroxide during the reaction.Essentially no chromium VI remains after the hydrogen peroxide reactionis complete, as indicated by a negative test using thes-diphenylcarbazide method (ASTM D 1687-02, Chromium in Water, TestMethod A), which produces a red-violet color in the presence of chromiumVI.

Mixtures of high-purity chromium III salts can also be made by the aboveprocedure, using mixed acids instead of a single acid such asfluozirconic. Formation of chromium III salts by the methods of thisinvention is believed to be general for a variety of salts, barring anyspecific detrimental interaction of the anion.

Using the methods of this invention, no mixing of separate trivalentchromium compounds (for example, CrF₃) with fluozirconic acid isrequired, and so no extraneous anions or cations are present in greaterthan trace amounts. The purity of the chromium III salt is determinedmainly by the purity of the raw materials used to prepare it. The salt,presumably Cr₂(ZrF₆)₃, functions well in pretreatment compositions andimproves the corrosion resistance of the substrate.

The methods of this invention make it possible to prepare chromium IIIsalts with very low levels of contaminating cations and anions. In someembodiments, the amount of any given contaminating cation (typically ametal, especially an alkali metal) is less than 1000 ppm, or less than500 ppm, or less than 300 ppm, or less than 200 ppm, relative tochromium. These limits may also apply specifically to the total of allalkali metal ions, or even to all metal ions in total.

Similarly, the content of contaminating anions may be very low, and insome embodiments the amount of any given contaminating ion is less than1000 ppm, or less than 500 ppm, or less than 300 ppm, or less than 200ppm, relative to chromium. In particular, these limits may applyindividually to each of fluoride, chloride, bromide, iodide, nitrate andsulfate. These limits may also apply to the total of all of theseanions, or even to all anions in total (including other anions notlisted here).

An alternative method of making the chromium III fluozirconate is tofirst produce hydrous chromium oxide, sometimes referred to as hydratedchromium oxide or hydrated chrome oxide. This material, approximately ofthe formula Cr₂O₃.nH₂O or perhaps Cr(OH)₃, can be made by reaction ofCr₂(SO₄)₃ (chrome alum) with a base such as NaOH. This approach requireswashing out or otherwise removing the resulting Na₂SO₄ byproduct fromthe resulting hydrous chromium oxide so as to produce a productessentially free of contaminating cations and anions. Once such washingis complete, the hydrous chromium oxide is contacted with fluozirconicacid to form aqueous high-purity chromium III fluozirconate.

In all of the above methods, the molar ratio of zirconium to chromium inthe final product will typically be about 1.5:1, as indicated by thestoichiometry implicit in the formula Cr₂(ZrF₆)₃. This calculates to aweight ratio of 2.63:1. The composition need not contain onlyCr₂(ZrF₆)₃, however. Thus, the weight ratio can vary somewhat from thestoichiometric value. The ratio will typically be at least 2.4:1, moretypically at least 2.5:1, and most typically at least 2.6:1. The weightratio will typically be at most 3.0:1, more typically at most 2.9:1, andmost typically at most 2.8:1. These ratios may be converted to theirequivalent mole ratios, and the corresponding weight ratios may then becalculated for Cr₂(TiF₆)₃ and Cr₂(HfF₆)₃.

The purity of the chromium III fluozirconate provided by the inventionmay be very high, even of reagent grade. High-purity chromium IIIfluozirconate is suitable for use in any application for which the useof chromium III fluozirconate is known, for example in electroplatingbaths.

Metal Treatment Formulations Comprising High-Purity Chromium IIIFluozirconate

Chromium III fluozirconate prepared according to the invention may beused to treat metal surfaces to improve corrosion resistance. Inpractical use, the absence of extraneous cations and anions provides asignificant advantage. This is because chromium III fluozirconatesolutions made by mixing CrF₃ with fluozirconic acid, for example,contain extraneous fluoride ions that continue to build up inconcentration as the treatment bath is replenished with additionalchromium III fluozirconate solution, as it is consumed by the metalbeing treated. Such buildup can cause the treatment coating to becomeweak and can also cause sludge to form in the bath. These problems maybe practically eliminated by instead using the high purity chromium IIIfluozirconate solutions of this invention. Further, the high puritymakes it possible to merely dry the solution in place on the treatedmetal surface without rinsing, if so desired, thereby saving a step andreducing waste handling.

Any metal may be treated, with particularly good results being obtainedon zinc, zinc alloy, aluminum, and aluminum alloy surfaces. As usedherein, the term “zinc alloy” means an alloy in which zinc is present ata weight percent greater than that of every other metal in the alloy.The term “alloy” is similarly defined for every other metal to which theterm is applied. Metal alloys typically contain at least 50 wt % of thenamed metal.

The chromium III fluozirconate may be applied alone from aqueoussolution to a metal surface and subsequently dried to remove water,resulting in effective passivation of the surface. Alternatively, thechromium III fluozirconate may be mixed with an organosilicon compoundsuch as aminopropyl triethoxysilane to make a metal treatmentformulation. The addition of organosilicon compounds may improveadhesion of subsequently applied coatings (such as paints) to thetreated surfaces, while maintaining good corrosion resistance in aNeutral Salt Spray test.

The pH of metal treatment formulations according to the invention willtypically be at least 2.5, more typically at least 3.0, and mosttypically at least 3.5. The pH will typically be at most 5.5, moretypically at most 5.0, and most typically at most 4.5.

EXAMPLES Preparation of High-Purity Chromium III Fluozirconate Example 1

Aqueous chromic acid was placed into a beaker and fluozirconic acid wasadded. Hydrogen peroxide was then added with stirring until completereduction of chromium was indicated by a very sharp color change fromyellow-green to blue-green. A test for hexavalent chromium usings-diphenylcarbazide (a reagent specific to hexavalent chromium capableof detection of hexavalent chromium at levels at sub-micrograms perliter levels) was negative, confirming complete conversion to trivalentchromium.

Specific quantities are as below:

chromic acid (nominal 40%, aqueous)—36.2 g

fluozirconic acid (nominal 45%, aqueous)—100.0 g

hydrogen peroxide (nominal 35%, aqueous)—24.6 g+5.0 g extra

Example 2

A mixture of chromic acid and fluozirconic acid was provided bydissolving chromium trioxide (reagent grade) in aqueous fluozirconicacid, and hydrogen peroxide was then added while mixing at a high rateto reduce the hexavalent chromium to chromium III. Completion of thereduction was signaled by the change in color from yellow-green toblue-green, and confirmation of the conversion was bys-diphenylcarbazide.

Specific Quantities:

chromium trioxide (CrO₃)—93.8 g

fluozirconic acid (45%, aqueous)—648.1 g

hydrogen peroxide (35%, aqueous)—258.1 g+23.5 g extra

Example 3

BULK RINSE® 6A, a partially reduced chromic acid, was added tofluozirconic acid, and sufficient hydrogen peroxide was then added tocomplete the reduction of chromium VI to chromium III. The color onceagain indicated completion of the reduction, which was again confirmedby s-diphenylcarbazide addition.

Example 4

A large-scale preparation of Cr₂(ZrF₆)₃ was prepared in a manneranalogous to Example 2 above, using the following amounts ofingredients.

chromic acid liquid (40%)—56.7 lbs

fluozirconic acid 156.3 lbs

hydrogen peroxide—62.2 lbs

Reduction of chromium VI to chromium III was complete well before all ofthe hydrogen peroxide was added, but the rest of the hydrogen peroxidewas still added. Apparently, this run did not require as great an excessof hydrogen peroxide as for the smaller scale preparations, although thereason for this is not completely clear.

Metal Treatment With High-Purity Chromium III Fluozirconate

The effectiveness of high-purity chromium III fluozirconate forpassivating aluminum 2024 and 6061 panels was investigated according toU.S. Military specification MIL-DTL-81706B, dated 25 Oct. 2004. Inparticular, see reference to Type 2 non-chromate products. The treatmentprotocol was as shown below, and is similar to that disclosed in U.S.patent application Ser. No. 12/116,513 filed May 7, 2008, the entiretyof which is incorporated herein by reference.

-   -   1) Clean—BULK KLEEN® 737G non-etching silicated alkaline cleaner        (Bulk Chemicals, Inc.), 15 g/L, 140° F., 5 minutes    -   2) Rinse—Tap water, ambient temperature, 30 seconds    -   3) Deionized water rinse, ambient temperature    -   4) Chromium III treatment: panels immersed at 100° F., 5 minutes        (See below for details of treatments)    -   5) Rinse with deionized water: ambient temperature, 30 seconds        (for seal application the two steps below are included)    -   6) Seal; E-CLPS® 1900 chrome-free pretreatment (Bulks Chemicals,        Inc.), 2% v/v dilution, ambient temperature, 30 seconds    -   7) Dry: 130° F. oven, 5 minutes

A chromium III composition was prepared, including the followingcomponents:

Constituent Wt. % 45% Fluozirconic Acid 4.27 Chromium (III) FluorideTetrahydrate 1.05 Ammonium Bifluoride 0.0125 Aminopropyl Triethoxysilane4.78 Water balanceThis composition, which had a Zr:Cr weight ratio of 2.80:1, was dilutedwith water to 4% of its original strength before being used to treataluminum panels, as a control.

High-purity chromium III fluozirconate was prepared according to themethod of Example 2, and supplied as a 37.9% stock solution. Thesolution was determined to have the following composition by weight,based on stoichiometric calculations using Cr₂(ZrF₆)₃ as the compoundformula: 5.47% Cr, 14.42% Zr, 8.0% F and Zr:Cr=2.6:1 by weight. Thisstock solution was used in the following examples, diluted as indicated.In each case, treated panels were exposed to Neutral Salt Spray (NSS)for 336 hours according to ASTM 8117, then rinsed and evaluated forpitting.

Example 5

A 0.121% dilution of the high-purity chromium III fluozirconate stocksolution in deionized water was prepared, yielding the same level of Cras in the control. This solution was adjusted to pH=4 using ammoniumcarbonate. Aluminum 6061 and 2024 panels were prepared using thetreatment protocol shown above, with and without the E-CLPS® 1900chrome-free pretreatment seal step. Following NSS, no pitting wasobserved for the panels treated with high-purity chromium fluozirconateor the control panels. Thus, the high purity chromium III fluozirconatesolution produced good results while avoiding the presence of extraneousfluoride ion, such as was present in the control formulation.

Example 6

a) A 2.21 g/L bath of the high-purity chromium III fluozirconate stocksolution was prepared and 1.90 g/L of SILWET® A1100 aminopropyltriethoxysilane (available from Crompton Corporation of Greenwich,Conn.) was added to yield a similar amount of silane as in the controlformulation. When heated to 100° F., this solution was hazy.Fluozirconic acid (1.5 g/L) was added to this bath to reach a pH of 4.0.While not as hazy, some floc was noted. The Zr to Cr ratio for thissolution was 3.86:1. Aluminum 6061 and 2024 panels were prepared usingthe treatment protocol shown above, with and without the E-CLPS® 1900chrome-free pretreatment seal step. Following NSS, no pitting wasobserved for the panels treated with high-purity chromium fluozirconateor the control panels.

b) Another bath was prepared using 2.21 g/L of the high-purity chromiumIII fluozirconate stock solution and 0.22 g/l of H₂ZrF₆ heated to 100°F., and pH adjusted to 4.0 with ammonium carbonate. The bath was notedto be hazy. The Zr to Cr ratio for this bath was 3.0:1. Aluminum 6061and 2024 panels were prepared using the treatment protocol shown above,with and without the E-CLPS® 1900 chrome-free pretreatment seal step.Following NSS, no pitting was observed for the panels treated withhigh-purity chromium fluozirconate or the control panels.

c) Still another bath was prepared using a 2.21 g/L of the high-puritychromium III fluozirconate stock solution, and SILWET® A1100 1.0 g/L wasadded, resulting in a pH of 4.0 at 100° F. Aluminum 6061 and 2024 panelswere prepared using the treatment protocol shown above, with and withoutthe E-CLPS® 1900 chrome-free pretreatment seal step. Following NSS, nopitting was observed for the panels treated with high-purity chromiumfluozirconate or the control panels.

Example 7

A bath composed 0.22 g/L of the high-purity chromium III fluozirconatestock solution in water was adjusted with SILWET® A-1100 (1.0 g/L) tobring the pH up to 4.0. The resulting Zr:Cr ratio was 2.6:1 by weight.Aluminum 6061 and 2024 panels were prepared using the treatment protocolshown above, with and without the E-CLPS® 1900 chrome-free pretreatmentseal step. Following NSS, no pitting was observed for the panels treatedwith high-purity chromium fluozirconate or the control panels.

A final modification was made by adding 0.22 g/L of H₂ZrF₆ to the abovebath to give a Zr to Cr ratio of 3.0:1. SILWET® A1100 (0.7 g/L) wasfurther added to bring the pH of the bath to 4.0. Aluminum 6061 and 2024panels were prepared using the treatment protocol shown above, with andwithout the E-CLPS® 1900 chrome-free pretreatment seal step. FollowingNSS, no pitting was observed for the panels treated with high-puritychromium fluozirconate or the control panels.

As the foregoing examples show, high-purity chromium III fluozirconatewas effective as a standalone passivant for aluminum at a pH of 4.0 andat the concentrations tested, with or without subsequent sealing withE-CLPS® 1900 chrome-free pretreatment. The skilled person willappreciate that, depending on the type of metal being treated,optimization of pH, temperature, exposure time, Zr:Cr ratio, and/orchromium III concentration may be adjusted to provide the bestperformance.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. A method of treating a metal surface, comprisingcontacting the metal surface with an aqueous composition comprisingdissolved Cr₂(GF₆)₃, in which G is a Group IV-B element, and containingless than 500 ppm in total of alkali metal ions and less than 200 ppm ofhalide ions, relative to chromium.
 2. The method of claim 1, wherein theaqueous composition contains one or more contaminating ions, and whereineach of said one or more contaminating ions is present at less than 1000ppm relative to chromium.
 3. The method of claim 1, wherein G iszirconium.
 4. The method of claim 3, wherein the aqueous composition hasa weight ratio of Zr:Cr in a range from 2.4:1 to 3.0:1.
 5. The method ofclaim 1, wherein G is titanium.
 6. The method of claim 1, wherein themetal surface is Al, Zn or an alloy of either of these.
 7. The method ofclaim 1, further comprising drying the composition in place on the metalsurface without an intervening rinse step.